If you need a special size/shape oven build it. If you want a reliable oven with time tested controller made to do knives, buy what you need.
A factory oven costs roughly double what a home built one costs. If you are handy and don't count your time as a cost, it is maybe only 1/3 the cost of a pre-made oven.
It is also important to remember -
There is no warranty or tech support for a home built unit.
The people who build commercial ovens ( Paragon, Evenheat, etc.) have built tens of thousands of them. They have already gone through the learning curve and made improvements. They have the right parts, the right controllers, and the right software installed.
I have seen some nice home builds, but none really looked or worked like a factory unit.
If you get tired of making knives or doing HT, you can sell your factory unit for a reasonable percentage of the cost. Home built units often end up in yard sales.