Building a foley stage

My prayers were heard – this forum looks so promising! Being a sound designer but not acoustical engineer, I really need some help on this subject: I am planning Foley pits for our studio in Ljubljana, Slovenia (Europe… ;-)). We have a recording room for speakers which is more or less soundproof and acoustically not too bad (absorbers, no parallel walls, double-door… it got some treatment and is quite a good room). I don't plan to change the room acoustics, "all" I want to do is build some Foley pits. The room is ca 12 square meters big, 2.8 meters high and the floor is hard wooden parquet - luckily not hollow or squeaky. Anyhow, I cannot dig a hole in the ground, but will have to build some kind of podium, not higher than 18cm (ca 6 inch). Apologies here: sorry, but Europe is using the metric system… The goal is to build six pits with different grounds. I think of these grounds: concrete (or paving stone), wood (rough and creaky), sand, gravel, and two pits that can store a marble plate, a steel plate, a large ceramic tile, linoleum, some different thick rubber mats (for individual damping/sounding of plates put on them), and should be water resistant enough to do some "mud-fun" in it or whatever might come to mind. So far so good. The biggest challenge is, that this room will still mostly be used for recording voice talents, so the Foley pits need a of cover so talents don't fall or sink in the sand pit, plus (to make things difficult) every now and then there is a drum kit to be recorded as well... I hope that thick wooden covers (every pit will have its own cover) and a thick carpet to put over the pits while not in use will solve this problem. So, but here I need help: First: What materials should I use to make the pits sound solid? I thought about a wooden frame and rubber mat underneath the frame to damp the pits, and probably even rubber on the frames to uncouple one pit from the other. Aside of that I wonder how to protect the framing from moisture. The stage is not meant to be for water recordings, but things like a wet pavement and some mud should be possible. Second question: How do I build a wooden pit that will be squeaky? Third question: Is it necessary to fill the podium around the pits with cement, or could it be just a wooden framing (built with thick wood of course) and probably filled with something else for damping? Budget? Well, since I am new in Slovenia I don't really have an overview of material costs, but roundabout 1500€ should be max, and, honestly, I hope I can make it with half (material only, of course) Best Regards from Ljubljana Johanna