You can do Tom Dixon's kept foam, use 3/8 in. wide cap strips, and cut the foam out of the bays. Be sure to leave at least a 1/2 in. thick strip of foam between the cap strips. Foam isn't quite as stiff as balsa and needs to be proportionately thicker for strength. I've done it on sport/combat style planes. Unfortunately, the balsa and glue add almost as much weight as the foam removed. Does look nice though, since it looks like standard rib construction
This approach is notorious for the aft part of the wing collapsing. Todd Lee's older silver Mustang/Mustang had this problem as did Ted's "Russian" airplane, and a bunch of others locally. It doesn't make much difference in the performance. Todd and his collapsing airfoil beat me and my not-collapsing airfoil handily, and was very close to winning the entire thing, at the 2000 NATS and qualified easily for the 2001 WC Team.
Last time this came up, and I told people not to do it that way, I got a nasty-gram from Tom saying that he *does not* recommend cutting out the foam in the bays - which is certainly going to help the "collapsing aft airfoil" problem. It does have a potential problem with the tissue or covering getting stuck to the foam in between the capstrips but I would expect that just being careful would solve that problem.
I might also note that this is not a new idea. I have an old issue of AAM from maybe about 1972 that shows an advertisement for a Nobler wing built very similarly.
Brett