First, the critical part is to ensure the cup fits the expanded glans as closely as possible. If the 1.5in diameter cup is too snug, the next one up is 2in. That will gave tons of gap.
@DLD needs to contact the manufacturer to obtain an in-between size of 1.75in cup so the brothers don't have to play blister-buster.
Second, if you have gaps in the cup due to the cup size, you can use the SiliCap but at the same time, make sure the gap is filled as much as you can. You can use felt fabric, or cushion materials similar to the soft TheraBand composition. I have medical grade foam that is a great help as well. Both can create a tighter cushion space to minimize gap between your glans and the cup's wall, and to prevent slippage.
Third, DO NOT and I heavily emphasized the DO NOT part in increasing the pressure. 1/2 twist max. Since the loose cup can cause slippage, we tend to increase the twist to 1.5 and 2 rotations. The worst you can ever subject yourself to as the increase pressure sends the internal vacuum pressure load to soar past -8inHg. BLISTER CITY if you're not conditioned for it.
Contact
buymos@gmail.com and request a custom invoice for both the SiliCap2 and the strap separately. It'll be much easier on your sanity.