Welcome to the forum MRS.G!

DIY diffusers are NOT where I would try to save the money, but there are those that have made them with soaker hose and/or PVC pipe. The small size of the bubbles is what makes a professional diffuser more desirable than a DIY model. Smaller bubbles yields more water movement. Making the stands for the diffusers will save some money however. Any savings from a DIY system is going to come from designing your own system, sourcing your pump (and shopping around), buying the components, and using your labor to install it (this includes the electric run or airline run to the pond). If you just look at the component costs...it's hard to save much money when compared to buying a turn-key system of like components. The real savings is in the labor to design and install. I am confident that the forums can help a lot with the design, but the labor is up to you. IIRC, I saved about $700 on my 1/4 horse system, but I had to run electric 175' underground to the pond which accounted for most of the savings as the neighbor and I did it for return favors. The rest of my savings was in my "pack-rat-ery". I had most of the plumbing parts and materials for the little pump house. I'd say I spent $1300 on my system which would aerate a 1 acre pond if needed even though my pond is only a 1/4 acre.

The first thing to do is to show us it's shape (with some good dimensions) and some depths. We are looking for the surface area and average depth...This will allow us to estimate how much water you have in the pond and how many diffusers you should have. From there, a pump size can be determined. The rest is pretty straight forward.

Fish on!,
Noel