I want to be a “man of my word” by fulfilling the promise I made to Greg to explain the process of veneering and a little bit about the bass trap, in hopes that it could be of help to others. I hope this isn’t too elaborate!
After picking 2x lumber for straightness and rounded edges, I start by picking the best edge for the front that will be veneered. To make sure it would have a good corner for bending veneer around, I routed the edge to a 3/16” radius. If I had it to do again, I would go with ¼” radius. Rolling the birch veneer over the 3/16” edge was pushing it a little bit too tight and I got a few spots that were trying to split.
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I cut a whole bunch of ¼” spacers and spray painted the edges with flat black so they wouldn’t be visible inside the Hemholtz slots. Then cut each board to length and painted the sides black. The idea is that I didn’t want the end of the veneer to show in the slot and black has a way of making it “invisible”.
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I used 2 ½ sheets of birch veneer in this bass trap (4’ x 8’ sheets). I calculated the widths of the veneer strips to allow 1” depth penetration into the slats and ½” penetration between boards with no slats. This meant I had to cut two widths: 3” and 5” because some boards are offset by 2" deep.
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I drew center lines on the edge of each board and a corresponding alignment line on each strip of veneer, so the overlap would come out right. Also, I marked and taped off the areas on each board where the contact cement would go. I trained my young assistant to glue a few boards and veneer strips at a time (while promising his parents to be practicing social distancing and taking COVID-19 precautions).
Make sure a medium-thick layer of glue is applied. The boards and the veneer can absorb the glue and it can get too dry. If that happens, you may have veneer coming loose later and will have pains trying to repair it.
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I carefully labeled each board and strip pair and kept track. The overlap is different from piece to piece and it gets confusing to keep track. Give the contact adhesive about 20 to 30 minutes to dry to the point that it is pretty tacky/dry.
When placing the veneer, it is important to get it right the first time, since the contact cement isn’t very forgiving and wants to stick immediately (of course). Back in the day, I used to lay out a bunch of dowels across a glued up table top and carefully placed the veneer sheet on the dowels. Then I could align the veneer up and start pressing it down from the middle working outward while pulling the dowels away. In this project, I have board EDGES – a bit different. After scrounging around for a method, I found a big coil of #4 insulated wire and cut a whole bunch of pieces and shaped them to go over the edge and placed them like this (the board is glued and ready):
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Then, with help from my assistant, we laid the strip over the board and lined up the alignment marks:
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Starting from the middle, working toward the ends, I press the veneer down on the edge while sliding the green wire spacers aside until the whole strip is down (while my assistant is keeping his end in alignment). The next step is to take a piece of 2x4 block with a “friendly” rounded edge and press firmly up and down the veneer with a lot of weight to get it firmly tacked down. Then carefully start forming it around the edge to the sides with the block, almost like ironing. Put a lot of good pressure to make sure it’s tacked down. Next, trim the ends with a razor knife and sand the trimmed ends slightly to make them round and smooth, so they don’t “catch” and tear.
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Briefly sand the veneer with an orbital sander and then block sand by hand. Be brief, because it is easy to sand right through the veneer. Sanding makes all the difference in the world to the look of the finish later. It brings out the beauty of the grain. I learned that working in a furniture refinishing shop.
I finished by brushing with 3 coats of oil based spar urethane in this case because it turns a little yellow. I wanted the “golden” color that looks good on birch. Sand between coats with 400 grit and wipe down with a tack cloth. It really helps to smooth out the finish that way.
After that came the task to assemble each of three sections, dealing with a bit of warps and twists and tolerances that would add up as I assembled 40 boards from left to right.
Here is a partially-assembled section. By the third section, I was getting the hang of it!
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I feel assured now! 
It will be a while until the 3rd section goes in. I have to insulate and then start from raw materials on the lumber. I had to wait to see what the remaining heights and widths would be (because of tolerances adding and slanted top, etc) before I could start cutting out the lumber and veneer and such:
