Normally... Transmission Shops and AT Repair Specialists use a Wide Table Top with a Sturdy Base sporting an Aluminum covering to avoid absorbing all of the Spilled ATF... and they employ the use of Transmission Holding Fixtures like THESE:
As an alternative, if you have any spare 2" X 4" sections laying around, perhaps you could be creative and design & build a Screwed-Together Base Support to be steady enough to do the Job after bolting the Case Flange up to it.
Mind you... if you do build such an apparatus, it can't be too tall and it must also be wide enough NOT to move around or tip over during this work.
Amazon.com : 4l60e transmission holding fixture
www.amazon.com
As an alternative, if you have any spare 2" X 4" sections laying around, perhaps you could be creative and design & build a Screwed-Together Base Support to be steady enough to do the Job after bolting the Case Flange up to it.
Mind you... if you do build such an apparatus, it can't be too tall and it must also be wide enough NOT to move around or tip over during this work.