I want to buy a large board of MDF 2440mm X 1200mm. I won't be able to fit it in my car so I will need to carry it on top. I was planning to put some towels on the roof and then hold it on to with loops of duct tape through the front and back windows. it then struck me that I won't actually be able to get in the car again if I loop it through both windows unless the front loop is just a single loop, which doesn't seem very secure. Whilst I'm only planning to travel about 3 miles, for safety reason I'm reluctant to do something that would render all 4 doors not easily openable in the event of an accident. I think it is probably best to have secured at 2 point. Duct tape at the back should stop it flying off the roof, but without anything else holding it in place I suspect it may be able to pivot left and right potentially weakening the tape. I'd be grateful for your suggestions. There will only be me and I'm too big to climb through car windows.
Best thing is DON'T. There are "fail" memes all over the internet from people trying to do this. It's neither safe nor effective. You should be able to get the piece delivered to your home at a reasonable surcharge -- which will certainly be less than the fine and liability you'll pay if the board comes off in traffic and damages someone else's property, or the repair bill when (not really if) you damage your own car.
If you can't get the board delivered, see if you know someone who has a large enough vehicle to carry it safely, either fully enclosed in a van body or at least tied down securely in a pickup truck bed.
I wouldn't recommend using tape, you're safest bet is to use a ratchet strap if you can get hold of one. You'll be able to secure the boards down and feed the strap through the doors, through experience this doesn't cause any issues with closing the doors and won't get in the way.
Reference: eBay Listing
Open the doors and loop the tape around through the doors, then close the doors on the tape. That will allow the doors to open. (Twine is another option, if you don't want duct tape adhesive on a lot of your car.)
Be aware of driving SLOWLY. A board like that, on your roof, can easily catch some wind and lift up.