I would say no don't do i.
The reason I would say this is because you need to ask the question in the first place, in my eyes that says your not sure of your ability.
You have to remember that this trailer will be carrying your bike and if it breaks at any point its very likely that at a minimum you will cause damage to the bike and who knows maybe someone else vehicle/property.
All it would take is for a single weld to fail say for example a rope tie down point. The bike could then topple over and your day will take a turn for the worse.
I have seen advertised here in Australia adaptors that convert a box trailer to carry bikes. Yes I know all these seem expensive but be serious about the cost of material and building cost, they are not as bad as they appear.
If your 100% sure of your ability to build a trailer, maybe go photograph and measure a few bike trailers and remember to look and photograph every bend in the steel and every weld.