I posted this on my blog as a three part tutorial, and thought that I should post it here as well. Slightly edited.
Czech out my hedgehogs!
Czech hedgehogs are anti-tank obstacles made of L or H shaped beams, and were used in huge numbers during WWII.
Even when tipped over they still function, and if you're dumb enough to try and tip them over with your AFV, you're essentially stuck.
German illustration of tank traps.
Image from missinglynx.com
With todays tactics of layered / staggered defence I guess they're more or less out of fashion. But they do look awesome on the 40K
My suggestion is to count these as impassable for vehicles, difficult terrain (dangerous if you add barbed wire) for infantry with a 6+ save.
The L-beams don't give much cover, just look at Saving Private Ryan
(@4:40 in that video) if you doubt that.
A note on the hedgehogs you see in SPR. They don't have the end-pieces on because they were on concrete "feet" in order to keep them from sinking in the sand.
If you plan to place hedgehogs on fairly solid ground you weld endpieces to them.
Now, Games Workshop makes a bunch of these, in the urban accessories set
The problem is that they're completely worthless. You do not want spiked ends on these since that would mean that your one tonne beam-construction slowly would sink into the ground.
And from what I've heard, the molds are kinda crap.
So, onto building realistic hedgehogs for our realistic games with melta-/plasma-guns!
First cut three 60mm lengths of L-beam. 60mm is kinda the middle ground. Not too big and not too small.
If you want bigger obstacles to stop Baneblades or something, make them 80-90mm long and go up in thickness on the L-beams.
Next cut six square-ish pieces for the ends of your L-beams. They dont have to be perfectly rectangular and you can save some money by using a 0.75mm thick sheet instead.
Since we'll be using one of these to make the brackets for the L-beams. I opted to buy strips since it saves some time and can be used for other stuff.
Glue them onto the ends of your L-beams. You will be cutting and trimming these later.
So in order to do that without too much hassle, glue them now so the bond will be strong enough for some manhandling later on.
Now take your sheet of 0.75mm plasticard and cut two 12 by 12 mm
If you want bigger "Saving Private Ryan brackets" go a little bigger. 16x16mm or even 20x20mm.
Score them from corner to corner, then remove one quarter...
...like this. These will serve as the middle bracket for your hedgehog. You only need two of these.
Since I was making more than one hedgehog there are three in the picture (and a bunch more off frame).
By now the glue on the ends of your hedgehogs should've dried and you can carefully trim off the excess from the end pieces.
Keep the scrap if you plan to do more hedgehogs. Also cut notches 5mm in. These are for securing barbed wire to the hedgehogs and the hedgehogs to each other.
Glue one of the bracket-pieces on one of the L-beams. Be careful with positioning. You want it in the middle.
Do the same with the second L-beam and glue them together. This will make a cradle for the third and last L-beam. Wait a little for the glue to set.
And then glue the third piece on there! Now you have yourself a czech hedgehog! If you really want to go all out, this is the time to add rivets to the bracket.
That's way too fiddly for me, but if you have the time to do it. By all means do it. I'd recommend using 0.5mm rod and making the brackets a little bit bigger.
Here's a good reference picture.
Comparison with an Ultramarine.
Now while you make two more hedgehogs I'll prepare part two of the tutorial in which I'll talk about basing the hedgehogs.