For a simple set of liquid explosives, as described in other sources, you wouldn't need to go anywhere. With a little practice you could do it by hand under a blanket or jacket in a few seconds.
Trying frantically to set your shoe one fire is one thing. Slipping something into a bottle of Gatorade is another. Once a device like that gets on a plane, it's pretty much over. Even if you fail to pull it off, you're not going to attract any attention, as opposed to Richard Reid, who got beat down as soon as the other passengers saw him take a match to his Nikes.
We also know now that there were three bombs per plane planned. So they had backup, if one didn't work.
The first time I flew after 9/11, I had a bottle of water with me. The screener had me open it and take a drink from it. That would be bad news for someone with part of an 'explosive cocktail' in their drink wouldn't it?