Spraymount is fine I think, but jam is traditional in this forum to attach film - Ole mentioned it and then when I was about to shoot my first glass-attached film I found my spraymount was missing the nozzle so I used it too. Just dont use raspberry.... jam is actually very easy to reposition, so I have carried on using it.
If you are using 4x5 on larger glass it might be difficult to get it not at an angle I guess.
I intend to make my own plates soon, but havent tried yet - maybe next week. Silverprint SE1 emulsion is the only one I can get easily, but I was wondering about making up something. I found a local glass shop who cut plates reasonably cheaply in 2mm picture glass (not very accurate, but you can sand them a bit and that gets rid of the sharp edges, and exact size is not that important).
Old plates are available in some sizes, but not usually as large as 13x18 - 4x5, 6.5x9 and 9x12 are the most common. I only recently got a 6.5x9 plate camera so havent tried this yet.