I use Giftster for this. It’s free and you can add items from anywhere even without a link. Also let’s you just take a photo of something and slap it on there.

I’ve used a cheap 3D pen to weld smaller prints together. Works pretty well. You have to be quick because once the filament starts to cool it doesn’t bind well. It also doesn’t look that pretty, but I only use it in areas that aren’t visible in the finished product. If you’re planning on sanding it after I think you’d be fine.