Yep, I do allow PCs to create non-slotted wonderous items. However, as a DM I'm working hand in hand with the PC to keep the proposed item balanced, anyway.
That and to be non-slotted, the item has to not occupy an already restricted slot; so no "it's a cloak, but it doesn't take the cloak slot". A cloak is a cloak, and cloaks take the cloak slot.
Ioun stones, musical instruments, figurines, tattoos and the like would be examples of non-slotted items. Alot of 'em fall into the category of Thing You Hold, rather than Thing You Wear.