eddybear2016Ultimate Guard Boulders (80 and 100 sizes) do not fix in this box. You're best off using an Ultimate Guard Arkhive 400+ as they are designed to fit 5 80+ or 4 100+ boulders like a glove.
Ultra Pro boxes (the soft plastic single deck ones) should fit, but this case is more designed for a cube/row of sleeved cards. More of a huge deck box in itself.