Here's a link to a tutorial for making box envelopes
I use the 1/4" envelobox on the original ultimate and use double sided tape to seal them - If I have to rely on a bought envelope for an A4 card I apply extra lick & stick remoistenable envelope glue along the original gum line and it works perfectly and is very secure.
I don't have much luck with boxes, they always seem to gape along the longest sides - any tips?
I also buy box envelopes for my 6 x6 cards but I get them from Ebay, I think they work out a little cheaper than the Glitterpot and definitely cheaper if you have to addd P&P. Not sure about the comparison in quallity although they look exactly the same. Not sure if I can put the link on here? If some kind person will tell me I can then I will, if not PM me :)