I needed a small slab mold for a soapmaking challenge two years ago, but didn't want to pay the high prices for what was out there, so I shopped around and found
this wooden box at Target that I line with freezer paper to create a perfectly suitable slab mold for small batches. I don't think it's as big as you are looking for, but it gives you an idea of alternatives to the pricier molds out there if you aren't sure you're going to really like making soap in a slab mold. Some soapers use slab molds frequently. Others only rarely - I'm in the rarer category.
Here is a Crafter's Choice 10" square slab mold that I think makes about a 3-pound recipe. I show it because smaller slab molds are less available than the larger ones for 5 or more pounds of soap. Although there is also
this one on Etsy, but to me it doesn't look as if it is as sturdy as the Crafter's Choice molds. But of course, since I don't have them side-by-side to compare, I don't really know. But the CC mold is actually a better price.
I own the Crafter's Choice one-pound silicone mold and can verify that it is really well constructed to holds its shape and not bow out from the weight of the soap. I am not sure how well constructed the 10" square one it, but I suspect it also has pretty rigid sides.
I have a long wooden mold with a very thin, rather flimsy silicone liner that sags when there is no soap in it, so it's a sort of a pain keeping the liner out of the way while pouring the soap. (I tape it to the wooden box with painters tape to keep it out of the way.) But I only use that mold when I am making a very large batch of soap. I think its like 5 or 6 pounds of soap (I don't remember as I haven't used it in a while.) I mention this because it is something to keep in mind if you look at wooden molds with those very thin silicone liners. The thin liners are easier to remove from the soap (easier than the thicker liners), once it is removed from the wooden box, but it can be a pain when pouring the raw soap into the mold as I mentioned.
I don't like taking a mold apart to get the soap out either, but I do have one I bought last year from another soaper. It's the
18-bar birch wood mold from Bramble Berry with inserts. It's rather a pain taking apart and putting back together, but it serves a purpose and is useful for some soap designs. And I do like the fact that it has it's own lid and since I bought it very gently used, I felt I got a decent bargain and did not have to pay shipping. Still I'm glad I didn't buy it right off the bat because slab mold soaps aren't something I make very often.