How do I fix an irregular hole in my heater closet?
You'll need to square up the hole and do some careful measuring and cutting, but it can be done:


On the floor, mark a rectangle that encloses the existing hole.  Ideally, the two pipes would be parallel to an edge, but it's not essential.
Measure the distance from the two closest corners to each of the pipes.
On some plywood, mark a rectangle the same size as the rectangle you marked on the floor.
Add 1/2 the outside diameter of the pipe to the distances you measured in step 2.  Using a drawing compass, mark an arc with the appropriate radius from each of the corners.  Where two arcs intersect will be the center of the pipe.
Using the drawing compass, mark a circle the outside diameter of the pipe.
Mark a line across the width of the rectangle going through the center of the two pipes.
Start cutting: cut out the rectangle on the floor, cut out replacement rectangle, cut along the line through the center of the pipes (this needs to be a thin saw blade), then cut out the four semicircles.
Cut out the rectangle on the floor.
If you've got room under the floor, you can use some wood under there as strapping to help hold the replacement pieces in place.  If not, you'll just have to do the best you can with wood glue.
Put the two pieces around the pipes; they should be able to slide up and down.  Again, if there's room under the floor, use a piece of wood as strapping to hold them together; if not, glue and hope for the best.
Slide them down into the hole in the floor.  They should fit snugly into place.  Screw them into the strapping if you were able to install some, otherwise just glue them in place.