With only five traps, there are two backroutes, one Cheapo-specific and one not. In the current version, the hidden sixth trap kills the CustLemm backroute but leaves the Cheapo-specific one in, so in the CustLemm version you could leave the level as it is.... I just think I prefer not to rely on hidden traps.
First backroute: compress all but one lemming, build over first two traps with one bridge, then over third and fourth with another. Release the crowd so that they pass the last trap while it's occupied killing the builder.
Cheapo-specific backroute: compress all but one lemming, have him bash to bypass the first trap, use two builders to get past the other four AND the hidden trap.