Is it really necessary to use a oven with spread on, would room temp 70-72* work if in for 2-3-4 or what ever days. I hate to build a oven for 1 bow, if I decide to make more I will build one. I still want it to be right.

Smooth on will self cure in 24 hours, although it may not have the resistance to more extreme temps later on.  I usually leave my leftover smooth-on in the mixing cup on my workbench, so that I can varify my mixture was correct.  Within 24 hours the epoxy was as it should be, and the temps were in the 50's.  That being said, you can build a simple box that will raise the air temp to the 125 degree range with stuff around the house.  To do my tip overlays, I used a cardboard box lined with a sheet of aluminum foil, with a foil lid and an old drop light with a 100w bulb.  I adjusted the foil cover so that it maintained about a 110 degree temp, and let it cook for 6 hours.  Position your light to provide indirect heat.  Just ramblins of an amature.    

I love it...the "run whatcha brung" way of gitten her done.  I have been whining about the money to build a hotbox myself to my wife (excuses) and I think I can rummage up a droplight or even two to bake a longbow.  SWEET!

I used to bend some card board up and clamp it around some wood laminated bows. a drop light or two and some old towels for a bit of insulation and I was good to go. Just use your nogging and don't catch stuff on fire. not sure the towels mattered. It works.

If you are curing a takedown limb you could use a large ice chest, just invert it and put a droplight under it.  You probably should just bite the bullet and build an oven, you are going to build more than one.  Most likely a whole bunch more than one, no such thing as building only one.