I'm so glad our nap is gone. Months of waking crying from his nap and being a grouch most of the afternoon to only brighten up just before BT!
Sorry, not much advice, I don't plan DS's catch up naps he just takes one in the car if/when he is tried enough to nod off in the lulling environment, heater on, movement, radio... i give him 10 or 20 mins, helps if I wake him and say we are at the4 shop rather than at home though, home = upset, shop = fun.
But, what about an extra snack in the afternoon, maybe cake (some sugar energy) and a drink to help him through to BT? I did do one or two EBTs at 5.00 but our night is not as long as yours (6am to 6.30am WU)
If you feel like experimenting, DS did his total drop after a 2.5hr nap when he was ill so maybe do one longer nap every couple weeks instead of a shorter one every 3 days? Just a random, out there, idea.