I'm paining to make a custom miner from scratch
(have experience with micro controlers and fpgas)
BTW thank you for your info if anyone reading has atempted to do so, have you succeeded?
Then
read the to post that is pinned to this section Mainly point-3. FPGA's are useless for mining BTC. Even today's 100THs ASIC-based miners have a very very small chance of ever finding a block in the lifetime of any single machine.
As for folks making ASIC-based miners that use existing chips, just check the Hardware section. There are 2 folks doing it there.